1. Field of the Invention
The invention relates to a method for suppressing disturbances in a bipolar data stream in the case of large line lengths. The signals of the data stream are fed to a coarse adjustment and a fine adjustment in an equalizer and to a downstream level detector. From there they are fed back to the equalizer from the level detector via a control device. The invention also relates to a circuit configuration for carrying out the method.
Particularly in the case of stringent requirements on range for the reception of ISDN data, problems can arise in intersymbol interference, modulation of the amplitudes by power supply interference sources, interference from high-frequency sources and direct crosstalk from neighboring transmission lines. In addition, continuous operation without interruption of reception requires a low bit error rate (1E-7) and is therefore exposed to the full temperature response over its entire operating time. Moreover, the relevant standards permit variations in transmitted amplitude of up to 50% and a bit rate variation of +/-13%. At the same time, there is a requirement for the automatic capability to adapt to arbitrary cable lengths within the specified ranges. Depending on cable type, for example, in the T1 standard these can be between 0 and 2000 meters or between 0 and 2800 meters.
Since the signal-to-noise ratio worsens continually with increasing length of the transmission link, ever more stringent requirements have to be placed on the equalizer and/or the equalizer itself produces bit errors because of constant corrections in continuous operation.
It has been known to date to transmit and evaluate at the start of transmission standard signals which serve as training pulses or reference pulses. Such a method and the associated arrangements are described by T. Suzulei et al. in the article "Line Equalizer for a Digital Subscriber Loop Employing Switched Capacitor Technology"; IEEE Transactions on Communications, Vol. COM-30, No. 9, September 1982, pp. 2074-82. Such an arrangement is also known from U.S. Pat. No. 4,768,205.
Methods are also used which require a perpetually continuous correction, or which render restrictions necessary with regard to the amplitude variation and the tolerated noise.
Furthermore, methods are known which work with a multiplicity of detection levels and/or fixed detection levels. A method in which fixed detection levels are used for regulation is described by G. J. Smolka et al. in the article "A 384-kbit/s ISDN Burst Transceiver"; IEEE Journal of Solid State Circuits, Vol. SC-22, No. 6, December 1987, pp. 1004-10. It is disadvantageous in these last-mentioned methods that erroneous settings cannot be prevented in some circumstances, depending on the technology used. Moreover, the outlay on circuitry rises very sharply for longer ranges. Furthermore, in such cases the influence of pulsed interference sources or individual interference events is seen very clearly on the receiving path of the equalizer, and they lead to bit errors.